1. Field of the Invention
This invention relates to a thermal printing method and apparatus for effecting printing line by line by the use of a heat sensitive medium, such as heat sensitive printing paper or ink donor film, and more particularly to a method and apparatus for preventing sticks caused by cooling of the heat sensitive medium that occurs between successive printing of the lines.
2. Description of the Art
Thermal printing apparatus of a construction shown in FIG. 1 has been widely known. In this apparatus is provided a thermal head 1 comprising a number of heat generating elements. Upon application of image information for each scanning line to thermal head 1, the heat generating elements generate heat in accordance with the image information. Printing paper 2 having a heat sensitive color developing layer on one side thereof is pressed by a back roller 3 against thermal head 1 so that the heat sensitive layer is directly brought into contact with the heat generating elements of thermal head 1. In synchronism with the application of the image information to thermal head 1, the printing paper 2 is shifted in a direction (subscanning direction) perpendicular to the arranged direction (main scanning direction) of the heat generating elements, so that a desired printing is effected on the printing paper 2.
In this case, when a single heat generating element is considered, the element repeats generation of heat and cooling as shown in a temperature variation curve of FIG. 2(B) upon reception of a printing pulse voltage P1 as shown in FIG. 2(A). Whenever the temperature of the heat generating element exceeds a color developing temperature t.sub.a of the heat sensitive layer, printing of the information in a scanning line is carried out on the printing paper.
The heat sensitive color developing layer on the printing paper 2 is made of a pigment or a dye such as carbon black and a softening material such as wax having a melting point t.sub.b in a range of 60.degree. to 80.degree. C. Thus, when the printing pulse voltage P is applied to the heat generating element, the temperature of the element easily exceeds the melting point t.sub.b thus melting the layer temporarily.
However, during a time interval between two adjacent pulse voltages P.sub.1, the heat generating element is cooled to a temperature lower than the solidifying point t.sub.c, thus solidifying the melted layer causing the layer to stick to the contacting surface of the heat generating element. The stick of the solidified layer results in an increase of feeding load of the printing paper 2 and deterioration of the reproduced image.
Such a disadvantage not only occurs in the case of the above described direct type thermal printing apparatus, but also occurs in the case of a transfer type thermal printing apparatus. In this type of apparatus, an ink donor film 6 having a heat sensitive ink layer 5 formed on one side of a substrate film 4 as shown in FIG. 3, is used with a heat insensitive printing paper 7. Under the action of the back roller 3, the printing paper 7 is brought into contact with the heat sensitive ink layer 5 formed on the substrate film 4 which is fed in sliding contact with the thermal head 1 as shown in FIG. 4. When the temperature of the heat generating element in the thermal head 1 goes up in excess of a predetermined value under application of the printing pulse voltage P1 as described above, the ink in the heat sensitive ink layer 5 of the ink donor film 6 is transferred to the printing paper 7 for printing the image on the paper 7. However, since the substrate film 4 slidingly contacting the heat generating element is ordinarily made of a thermoplastic material, the substrate film 4 tends to be melted by the heat generated from the element. The melted film 4 is then solidified during the cooling period following the pulse voltage, the solidified film tending to stick to the surface of the thermal head 1, which not only increases the shifting load for the ink donor film 6, but also deteriorates the image quality. Such defects of the transfer type thermal printing apparatus can be easily understood by reading or interpreting the above described t.sub.a, t.sub.b and t.sub.c to be the transferring temperature of the ink, the melting point of the substrate film 4, and a the solidifying temperature of the substrate film 4, respectively.